ML20137E026
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r b k81 C N July 30, 1980 MEMORAEUM FOR: P. G. Shewmon D. Okrent FROM: H. Etherington
SUBJECT:
SEISHIC DESIGN
REFERENCE:
Memorandum, P. G. Shewmon to D. Okrent, dated June 18, 1980 and Memorandum, D. Okrent to P. G. Shewmon, dated July 3,1980 i I agree with the dissatisfaction expressed in both memorandums concerning the status of the piping system rigidity vs. flexibility controversy and believe j this subject, among other subjects, should be reviewed. In the initial layout of a piping system, the tendency of the designer is to ignore stresses other than thennal stresses, and to provide whatever' support is necessary to limit other piping stresses from external loading. Increased flexibility would therefore be achieved by reducing the stiffness of the supports and increasing the stress in the piping. Another effect of greater flexibility is to Tower the funda;nental frequency of the piping-support system, which could shift the syste:n response towards the high-displacement, low-frequency end of the seismic spectrum. Even though clastic theory gives an exaggerated picture of vibratory motion, I believe that elimination of this conservatis:n should be considered only in conjunction with other features of the design basis that may be less than satisfactory.
It seems to me that the present design basis is a fabric of unquantified con-servatisms, unconservatisms, and uncertainties in several areas; and also an inccasistent combination of stochastic and deterministic methodologies. Ef-forts are being made to improve the contlition in the individual disciplines, but perhaps the ACRS should sponsor a two- or three-day interdisciplinary seminar to define long range goals and near ter.n compromises. The ACRS has, in the past, sponsored seminars to guide the Committee in application and acceptance of new technologies and methodologies e.g., fracture mechanics, proababilistic methodology, and substitution of the analytical methods of Section III of the ASME Code for the se:ni-empirical rules of Sections Il and IV. Perhaps it is time to review the implications of the newer probabilistic approach'to mechanical design, either as an ultimate replacement for tradi-tional deterministic nethods or as a supporting procedure (Sec. 3 below).
In clarification of these views. I append an undisciplined and incomplete (possibly inaccurate) list of conservatisms and nonconservatisms.
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1 P. G. Shewmon U. Okrent l
- 1. Selection of the Design Earthquake-1.1 Unconservatisms 1.1.1 A substantial probability of exceedance of the largest historic earthquake during the 40-year reactor lifetime.
1.2 Conservatisms 1.2.1 Tranference of the epicenter to the nearest reasonably conceivable point to the site.
1.2.2 No deconvolution of free-field observations to the foundation.
1.3 Uncertainti_es .
1.3.1 Difficulty in assessment of newspaper accounts of historic ea rthquakes.
1.3.2 Difficulty of defining tectonic provinces and struc' tures.
F 1.3.3 Adequacy of design margins over the limiting historic '
1.3.4 Significance of phenoinena that have been intensively studied only recently (plate tectonics; magnetic, gravity, and radiation anomalics; microseismicity, etc.).
1.3.5 The possibility of unfavorabic post-construction dis-coveries (previously undiscovered faults, new signi-ficance placed on known faults such as the Hosgri fault and the Stafford fault zone, unsuspected soil proble.as such as the halloysite problem at North Anna, etc.).
- 2. Use of Regulatory Guide 1.60 2.1 Unconservatisms 2.1.1 Acceptance of spectra that have a 16 percent probability of exceedance.
2.1.2 The large standard deviation associated with 2.1.1.
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P. G. Shewmon , 3, D. Okrent . ,
2.2 Conservatisms ~
,2.2.1 Use of envelopes derived from a number of earthquakes each of which shows amplifications corresponding to the envelopes over only a limited range of frequencies.
2.3 Uncertaint'ies -
2.3.1~ Those stated in the caveats of Regulatory Guide 1.60 relative to soil conditions and geographic location.
. 2.3.2 Unsuspected unfavorable soil conditions.
2.3.3 Soil-structure interactions.
- 3. Mechanical Design 3.1 Unconservatisms .
3.1.1 Use of deterministic analysis, i.e., the assumption
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that a structure will not fail under design conditions, implying no margin of error (or a definite margin) with respect to design, material specification,
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material quality, fabrication. Installation, operation, or maintenance., ~
3.1.2 The exceedance p[obability implicit in use of SRSS to combine stresses. -
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l 3.2 Conservatisms -
3.2.1 With elastic theory. 'use of the Regulatory Guide 1.61 l
damping factors.
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- 3.2.2 Actual material properties are better than minimum specifications.
3.3 Uncertainties 3.3.1 Selection of appropriate ductility factors with i inelastic theory.
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NCC FORM 318 (9 76) NRCM 0240 N U.S. GOVERNMENT PRINTING OFFICE: 1979-289 369
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'P. G. Shest.aon 4 D. Okrent 3.3.2 Items in 3.1.1, which require, and are susceptable to, probabilistic treatment.
o H. Etherington t ACRS Member Emeritus cc: ACRS Members ,
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